Abstract: An in vitro fertilization method comprising culturing mammalian immature oocytes obtained from a mammalian ovary very early in the mammal's menstrual cycle in an oocyte maturation inhibitor-containing culture, removing the inhibitor and maturing and fertilizing the oocytes to produce embryos for subsequent uterine-implantation. The method allows for more convenient, efficacious, safer, less expensive and timely implantation of a plurality of embryos in the uterus to enhance pregnancy success rate. Cryopreservation of the immature oocytes offers enhanced benefits Preferably, the mammal is human and the inhibitor is dbcAMP.